An exhaust emission control apparatus includes a catalytic converter provided in an exhaust passage of an internal combustion engine to perform catalytic conversion of exhaust gases from the engine. The catalytic converter typically includes a catalyst and a heater for heating the catalyst to accelerate the rate of the catalytic conversion of exhaust gases when the engine undergoes a cold start. Electric power is applied to the heater of the converter for heating the catalyst in the catalytic converter.
It is necessary to operate the catalytic converter at high temperatures after a cold engine start in order to reduce the time for the catalyst to be activated. However, it is undesirable to operate at excessive temperatures for a long period of time since thermal deterioration is a significant contributor to a loss of monolithic three-way catalyst activity. This deterioration results in a decline in the conversion efficiency of the converter thereby reducing its effectiveness thereafter to achieve the desired performance. In order to prevent this deterioration in the catalytic converter, it is necessary to take corrective action when the temperature of the catalytic converter approaches an undesirable level that may result in its deterioration and therefore its performance.
The known prior art provides for sensing the temperature of the catalytic converter utilizing a sensor as well as estimating the temperature of the catalytic converter. One known prior art method of estimating the temperature of the catalytic converter is disclosed in U.S. Pat. No. 4,656,829, issued to Creps et al. The catalytic converter temperature is estimated based on engine operating parameters utilizing empirically determined steady state temperature contributions to the catalytic converter from the mass air flow through the engine and the air/fuel ratio of the mixture supplied to the engine. There is no specific compensation for the heat generated by the heater of the catalytic converter.